U.S. patent number 8,592,108 [Application Number 13/710,426] was granted by the patent office on 2013-11-26 for method for design and manufacture of patterns with variable shaped beam lithography.
This patent grant is currently assigned to D2S, Inc.. The grantee listed for this patent is D2S, Inc.. Invention is credited to Akira Fujimura, Harold Robert Zable.
United States Patent |
8,592,108 |
Fujimura , et al. |
November 26, 2013 |
Method for design and manufacture of patterns with variable shaped
beam lithography
Abstract
In the field of semiconductor device production, a method and
system for fracturing or mask data preparation or optical proximity
correction are disclosed, in which a target maximum dosage for a
surface is input, and where a plurality of variable shaped beam
(VSB) shots is determined that will form a pattern on the surface,
where at least two of the shots partially overlap, and where the
plurality of shots are determined so that the maximum dosage
produced on the surface is less than the target dosage. A similar
method is disclosed for manufacturing an integrated circuit.
Inventors: |
Fujimura; Akira (Saratoga,
CA), Zable; Harold Robert (Palo Alto, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
D2S, Inc. |
San Jose |
CA |
US |
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Assignee: |
D2S, Inc. (San Jose,
CA)
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Family
ID: |
42337223 |
Appl.
No.: |
13/710,426 |
Filed: |
December 10, 2012 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20130101941 A1 |
Apr 25, 2013 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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13300601 |
Nov 20, 2011 |
8329365 |
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12750709 |
Nov 22, 2011 |
8062813 |
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12540328 |
Sep 13, 2011 |
8017286 |
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12202364 |
Jul 20, 2010 |
7759026 |
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12473241 |
Jul 13, 2010 |
7754401 |
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Current U.S.
Class: |
430/5; 716/53;
430/942; 430/296; 430/30 |
Current CPC
Class: |
H01J
37/3174 (20130101); G03F 1/78 (20130101); B82Y
40/00 (20130101); G06F 30/20 (20200101); G03F
7/20 (20130101); B82Y 10/00 (20130101); Y10S
430/143 (20130101) |
Current International
Class: |
G03F
1/20 (20120101); G03C 5/00 (20060101) |
Field of
Search: |
;430/5,30,296,942
;716/53 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1992155337 |
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May 1992 |
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JP |
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11233401 |
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Aug 1999 |
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JP |
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2008066441 |
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Mar 2008 |
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JP |
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1020080001438 |
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Jan 2008 |
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KR |
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Other References
Bloecker, M. et al., "Metrics to Assess Fracture Quality for
Variable Shaped Beam Lithography", Proceedings of SPIE, vol. 6349
(Oct. 2006), pp. 63490Z-1-63490Z-10, SPIE, P.O. Box 10, Bellingham,
WA. 98227, U.S.A. cited by applicant .
International Preliminary Report on Patentability and Written
Opinion dated Mar. 10, 2011 for PCT Patent Application No.
PCT/US2009/053327. cited by applicant .
International Preliminary Report on Patentability and Written
Opinion dated Mar. 10, 2011 for PCT Patent Application No.
PCT/US2009/053328. cited by applicant .
International Preliminary Report on Patentability and Written
Opinion dated Mar. 10, 2011 for PCT Patent Application No.
PCT/US2009/054229. cited by applicant .
International Preliminary Report on Patentability and Written
Opinion dated Mar. 10, 2011 for PCT Patent Application No.
PCT/US2009/054239. cited by applicant .
Notice of Allowance and Fee(s) due dated Apr. 5, 2011 for U.S.
Appl. No. 12/473,248. cited by applicant .
Notice of Allowance and Fee(s) due dated Jan. 20, 2011 for U.S.
Appl. No. 12/473,265. cited by applicant .
Office Action dated Aug. 20, 2010 for U.S. Appl. No. 12/202,365.
cited by applicant .
Office action dated Mar. 17, 2011 for U.S. Appl. No. 12/540,322.
cited by applicant .
Office Action dated Mar. 18, 2011 for U.S. Appl. No. 12/540,321.
cited by applicant .
Office Action dated Mar. 2, 2011 for U.S. Appl. No. 12/987,994.
cited by applicant .
Office Action dated Mar. 30, 2012 for U.S. Appl. No. 13/316,564.
cited by applicant .
Office Action dated Mar. 31, 2011 for U.S. Appl. No. 12/540,328.
cited by applicant .
Chinese Office Action dated Jan. 14, 2013 for Chinese Application
No. 200980134188.6. cited by applicant .
Notice of Allowance dated Mar. 26, 2013 for U.S. Appl. No.
13/650,618. cited by applicant .
Office Action dated May 24, 2013 for U.S. Appl. No. 13/329,315.
cited by applicant .
Japanese Office Action dated Oct. 8, 2013 for Japanese Patent
Application No. 2011-525091. cited by applicant.
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Primary Examiner: Young; Christopher
Attorney, Agent or Firm: The Mueller Law Office, P.C.
Parent Case Text
RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser.
No. 13/300,601 filed Nov. 20, 2011 entitled "Method For Design And
Manufacture Of Diagonal Patterns With Variable Shaped Beam
Lithography" and issued as U.S. Pat. No. 8,329,365, which is a
continuation of U.S. patent application Ser. No. 12/750,709 filed
Mar. 31, 2010 entitled "Method For Design And Manufacture Of A
Reticle Using A Two-Dimensional Dosage Map And Charged Particle
Beam Lithography" and issued as U.S. Pat. No. 8,062,813, both of
which are hereby incorporated by reference for all purposes. U.S.
patent application Ser. No. 12/750,709: 1) is a
continuation-in-part of U.S. patent application Ser. No. 12/540,328
filed Aug. 12, 2009 entitled "Method For Design and Manufacture of
a Reticle Using a Two-Dimensional Dosage Map and Charged Particle
Beam Lithography" and issued as U.S. Pat. No. 8,017,286; 2) is a
continuation-in-part of U.S. patent application Ser. No. 12/202,364
filed Sep. 1, 2008 entitled "Method and System for Manufacturing a
Reticle Using Character Projection Particle Beam Lithography" and
issued as U.S. Pat. No. 7,759,026; 3) is a continuation-in-part of
U.S. patent application Ser. No. 12/473,241 filed May 27, 2009,
entitled "Method for Manufacturing a Surface and Integrated Circuit
Using Variable Shaped Beam Lithography" and issued as U.S. Pat. No.
7,754,401; and 4) is related to U.S. patent application Ser. No.
12/540,323 filed Aug. 12, 2009, entitled "Method For Design And
Manufacture Of A Reticle Using Variable Shaped Beam Lithography"
and issued as U.S. Pat. No. 7,799,489; all of which are hereby
incorporated by reference for all purposes.
Claims
What is claimed is:
1. A method for fracturing or mask data preparation or optical
proximity correction comprising the steps of: inputting a target
maximum dosage for a surface; and determining a plurality of
variable shaped beam (VSB) shots for a charged particle beam
writer, wherein the plurality of VSB shots forms a pattern on the
surface, wherein at least two shots in the plurality of VSB shots
partially overlap, and wherein the plurality of VSB shots is
determined such that a maximum dosage produced on the surface by
the plurality of VSB shots is less than the target maximum
dosage.
2. The method of claim 1 wherein the at least two shots partially
overlap within an exposure pass.
3. The method of claim 1 wherein the step of determining comprises
calculating the maximum dosage on the surface, and wherein the
calculating is performed by one or more computing hardware
processors.
4. The method of claim 3 wherein the maximum dosage on the surface
is calculated within an exposure pass.
5. The method of claim 3 wherein the maximum dosage on the surface
is calculated using charged particle beam simulation.
6. The method of claim 5 wherein the charged particle beam
simulation includes at least one of the group consisting of forward
scattering, resist diffusion, Coulomb effect, and etching.
7. The method of claim 1 wherein each shot in the plurality of VSB
shots comprises a dosage information, before proximity effect
correction (PEC).
8. The method of claim 7 wherein the step of determining comprises
calculating the maximum dosage on the surface, and wherein
calculating the maximum dosage on the surface comprises combining
the dosage information for the at least two partially-overlapping
shots.
9. The method of claim 7 wherein the step of determining comprises
calculating the dosage information for the least two
partially-overlapping shots using charged particle beam
simulation.
10. The method of claim 8 wherein the dosage information for the at
least two partially-overlapping shots is combined into a dosage
map.
11. The method of claim 7 wherein the dosage information for a pair
of shots in the plurality of VSB shots differ.
12. A method for manufacturing an integrated circuit using an
optical lithographic process, the optical lithographic process
using a reticle, the method comprising the steps of: inputting a
target maximum dosage for a reticle; determining a plurality of
variable shaped beam (VSB) shots, wherein the plurality of VSB
shots forms a pattern on the reticle, wherein at least two shots in
the plurality of VSB shots partially overlap, and wherein the
plurality of VSB shots is determined such that a maximum dosage
produced on the surface by the plurality of VSB shots is less than
the target maximum dosage; and forming the pattern on the reticle
with the plurality of VSB shots.
13. The method of claim 12 wherein the at least two shots partially
overlap within an exposure pass.
14. The method of claim 12 wherein the step of determining
comprises calculating the maximum dosage on the reticle.
15. The method of claim 14 wherein the maximum dosage on the
reticle is calculated within an exposure pass.
16. The method of claim 14 wherein the maximum dosage on the
reticle is calculated using charged particle beam simulation.
17. The method of claim 16 wherein the charged particle beam
simulation includes at least one of the group consisting of forward
scattering, resist diffusion, Coulomb effect, and etching.
18. The method of claim 12 wherein each shot in the plurality of
VSB shots comprises a dosage information, before proximity effect
correction (PEC).
19. The method of claim 18 wherein the step of determining
comprises calculating the maximum dosage on the reticle, and
wherein calculating the maximum dosage on the reticle comprises
combining the dosage information for the at least two
partially-overlapping shots.
20. The method of claim 18 wherein the step of determining
comprises the step of calculating the dosage information for the
least two partially-overlapping shots using charged particle beam
simulation.
21. The method of claim 19 wherein the dosage information for the
at least two partially-overlapping shots is combined into a dosage
map.
22. The method of claim 18 wherein the dosage information for a
pair of shots in the plurality of VSB shots differ.
23. A system for fracturing or mask data preparation or optical
proximity correction comprising a device configured to determine a
plurality of variable shaped beam (VSB) shots, wherein the
plurality of VSB shots forms a pattern on the surface, wherein at
least two shots in the plurality of VSB shots partially overlap,
wherein the plurality of VSB shots is determined such that a
maximum dosage produced on the surface by the plurality of VSB
shots is less than a desired maximum dosage.
24. The system of claim 23 wherein the device configured to
determine assigns a dosage information to each shot in the
plurality of VSB shots.
25. The system of claim 23, wherein the device configured to
determine calculates the maximum dosage on the surface.
26. The system of claim 25 wherein the maximum dosage on the
surface is calculated within an exposure pass.
27. The system of claim 25 wherein the maximum dosage on the
surface is calculated using charged particle beam simulation.
Description
BACKGROUND OF THE DISCLOSURE
The present disclosure is related to lithography, and more
particularly to the design and manufacture of a surface which may
be a reticle, a wafer, or any other surface, using charged particle
beam lithography.
In the production or manufacturing of semiconductor devices, such
as integrated circuits, optical lithography may be used to
fabricate the semiconductor devices. Optical lithography is a
printing process in which a lithographic mask or photomask
manufactured from a reticle is used to transfer patterns to a
substrate such as a semiconductor or silicon wafer to create the
integrated circuit. Other substrates could include flat panel
displays or even other reticles. Also, extreme ultraviolet (EUV) or
X-ray lithography are considered types of optical lithography. The
reticle or multiple reticles may contain a circuit pattern
corresponding to an individual layer of the integrated circuit and
this pattern can be imaged onto a certain area on the substrate
that has been coated with a layer of radiation-sensitive material
known as photoresist or resist. Once the patterned layer is
transferred the layer may undergo various other processes such as
etching, ion-implantation (doping), metallization, oxidation, and
polishing. These processes are employed to finish an individual
layer in the substrate. If several layers are required, then the
whole process or variations thereof will be repeated for each new
layer. Eventually, a combination of multiples of devices or
integrated circuits will be present on the substrate. These
integrated circuits may then be separated from one another by
dicing or sawing and then may be mounted into individual packages.
In the more general case, the patterns on the substrate may be used
to define artifacts such as display pixels or magnetic recording
heads.
In the production or manufacturing of semiconductor devices, such
as integrated circuits, maskless direct write may also be used to
fabricate the semiconductor devices. Maskless direct write is a
printing process in which charged particle beam lithography is used
to transfer patterns to a substrate such as a semiconductor or
silicon wafer to create the integrated circuit. Other substrates
could include flat panel displays, imprint masks for
nano-imprinting, or even reticles. Desired patterns of a layer are
written directly on the surface, which in this case is also the
substrate. Once the patterned layer is transferred the layer may
undergo various other processes such as etching, ion-implantation
(doping), metallization, oxidation, and polishing. These processes
are employed to finish an individual layer in the substrate. If
several layers are required, then the whole process or variations
thereof will be repeated for each new layer. Some of the layers may
be written using optical lithography while others may be written
using maskless direct write to fabricate the same substrate.
Eventually, a combination of multiples of devices or integrated
circuits will be present on the substrate. These integrated
circuits are then separated from one another by dicing or sawing
and then mounted into individual packages. In the more general
case, the patterns on the surface may be used to define artifacts
such as display pixels or magnetic recording heads.
Two common types of charged particle beam lithography are variable
shaped beam (VSB) and character projection (CP). In VSB charged
particle beam lithography, a precise electron beam is shaped and
steered so as to expose a resist-coated surface, such as the
surface of a wafer or the surface of a reticle. These shapes are
simple shapes, usually limited to rectangles of certain minimum and
maximum sizes and with sides which are parallel to the axes of a
Cartesian coordinate plane, and triangles with their three internal
angles being 45 degrees, 45 degrees, and 90 degrees of certain
minimum and maximum sizes. At pre-determined locations, doses of
electrons are shot into the resist with these simple shapes. The
total writing time for this type of system increases with the
number of shots. In CP charged particle beam lithography, there is
a stencil in the system that has in it a variety of shapes which
may be rectilinear, arbitrary-angled linear, circular, annular,
part circular, part annular, or arbitrary curvilinear shapes, and
which may be a connected set of complex shapes or a group of
disjointed sets of a connected set of complex shapes. An electron
beam can be shot through the stencil to efficiently produce more
complex patterns (i.e. characters) on the reticle. In theory, such
a system can be faster than a VSB system because it can shoot more
complex shapes with each time-consuming shot. Thus, an E shot with
a VSB system takes four shots, but the same E can be shot with one
shot with a character projection system. Note that VSB systems can
be thought of as a special (simple) case of character projection,
where the characters are just simple characters, usually rectangles
or 45-45-90 triangles. It is also possible to partially expose a
character. This can be done by, for instance, blocking part of the
particle beam. For example, the E described above can be partially
exposed as an F or an I, where different parts of the beam are cut
off by an aperture.
The photomasks used for optical lithography are manufactured from
reticles onto which a pattern has been formed. There are a number
of technologies used for forming patterns on a reticle, including
optical lithography and charged particle beam lithography. The most
commonly-used system is a VSB charged particle beam system. Reticle
writing typically involves multiple passes whereby the given shape
on the reticle is written and overwritten. Typically, two to four
passes are used to write a reticle to average out precision errors
in the charged particle beam system, allowing the creation of more
accurate photomasks. Conventionally, within a single pass the
constituent shapes do not overlap. Multi-pass writing has a
disadvantage of increasing the total time required for the charged
particle beam system to form the pattern on the reticle. This extra
time increases the cost of the reticles and the resulting
photomasks. At present, no available CP charged particle beam
system is suitable for use in making photomasks.
When using charged particle beam lithography either for making
reticles or for direct write, individual doses or shots of charged
particles are conventionally designed to avoid overlap wherever
possible, and for multi-pass writing, to avoid overlap within a
single pass. The dosage is assumed to be the same, or "normal," at
all parts of the formed pattern. This greatly simplifies
calculation of how the resist on the reticle will register the
pattern. Because of the assumed normal dosage, the fracturing
programs that assign VSB shots conventionally do not output dosage
information.
The cost of charged particle beam lithography is directly related
to the time required to expose a pattern on a surface, such as a
reticle or wafer. The exposure time is related to the number of
shots required to produce the pattern. Patterns can often be formed
in fewer shots if the shots are allowed to overlap. Additionally,
patterns can be formed in fewer shots if the union of shots is
allowed to deviate from the target pattern. When these techniques
are used, calculation of the pattern that will be registered by the
resist is more complicated. Charged particle beam simulation may be
used to determine the pattern that will be registered by the
resist. Charged particle beam simulation, which may include
simulation of various charged particle beam writing and resist
effects, is a compute-intensive process, however. It is impractical
to simulate the pattern for an entire integrated circuit, and then
to re-simulate the pattern every time a proposed charged particle
beam shot is changed.
It would therefore be advantageous to be able to easily determine
how resist on a surface such as a wafer or reticle will register a
pattern formed by a plurality of charged particle beam shots. This
would enable the use of overlapping shots and variable shot
dosages. With overlapping shots and variable dosages, patterns can
be formed on a surface with fewer shots, thus reducing the cost of
forming the pattern on a surface such as a reticle or a wafer, and
consequently reducing the cost of manufacturing photomasks and
semiconductor devices.
SUMMARY OF THE DISCLOSURE
A method and system for fracturing or mask data preparation or
optical proximity correction is disclosed, in which a target
maximum dosage for a surface is input, and where a plurality of
variable shaped beam (VSB) shots is determined that will form a
pattern on the surface, where at least two of the shots partially
overlap, and where the plurality of shots is determined so that the
maximum dosage produced on the surface is less than the target
dosage.
A method for manufacturing an integrated circuit is also disclosed,
in which a target maximum dosage for a reticle is input, and where
a plurality of variable shaped beam (VSB) shots is determined that
will form a pattern on the reticle, where at least two of the shots
partially overlap, where the plurality of shots is determined so
that the maximum dosage produced on the reticle is less than the
target dosage, and where the pattern is formed on the reticle with
the plurality of VSB shots.
These and other advantages of the present disclosure will become
apparent after considering the following detailed specification in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a conceptual flow diagram of a conventional
method for manufacturing a reticle and photomask;
FIG. 2 illustrates a conceptual flow diagram of manufacturing a
reticle and photomask using an exemplary method of the current
disclosure;
FIG. 3 illustrates a circular pattern, and an example of a dosage
map for a circular shot;
FIG. 4 illustrates a portion of a 200 nm diameter circular pattern
and dosage map, using a 4 nm grid;
FIG. 5 illustrates an exemplary dosage map for a rectangular
shot;
FIG. 6 illustrates a dosage map for a set of six overlapping
rectangular shots of the type of FIG. 5;
FIG. 7A illustrates a circular pattern;
FIG. 7B illustrates the dosage map of a rectangular shot;
FIG. 7C illustrates the dosage map of a square shot;
FIG. 7D illustrates the dosage map of three overlapping shots of
FIGS. 7B and 7C that can form the circular pattern of FIG. 7A;
FIG. 8A illustrates a parameterized glyph dosage map;
FIG. 8B illustrates another dosage map for the parameterized glyph
of FIG. 8A; and
FIG. 9 illustrates a dosage graph of a glyph resulting from a
circular CP character shot.
DETAILED DESCRIPTION OF THE EMBODIMENTS
FIG. 1 illustrates a conceptual flow diagram 100 of a conventional
method for making a photomask. The input to the process is a
computer representation 102 of a desired pattern that is to be
formed on a reticle from which the photomask can be manufactured.
In step 104 the pattern is fractured into a set of non-overlapping
shapes, such as rectangles and triangles, for exposure using a VSB
charged particle beam system. The result of step 104 is a shot list
106, in which the shots are non-overlapping. All shots are assumed
to have a normal dosage, and no dosage information is contained in
shot list 106. In step 108 proximity effect correction (PEC) is
performed, which assigns a dosage to each shot in the shot list,
and which may also slightly adjust the placement of the shots. Step
108 may also include other corrections which perform dosage
adjustments. The output of step 108 is a final shot list 110 which
includes dosage information. In step 112 a charged particle beam
system uses the shot list 110 to expose resist with which the
reticle has been coated, thereby forming a pattern 114 on the
resist. In step 116 the resist is developed. Through further
processing steps 118 the reticle is transformed into a photomask
120.
Variations of the FIG. 1 method exist. In one variation of this
process, called multi-pass exposure, the entire pattern is exposed
once, and then exposed a second time, called two-pass exposure.
More than two passes may also be used. Multi-pass writing may be
used to reduce non-ideal writing effects such as resist heating,
resist charging and field-to-field misalignment. In multi-pass
writing, the dosage for each pass is proportionately lower than for
single-pass writing, the goal being that the sum of the doses for
all passes will be a normal dose for all parts of the pattern.
Conventionally, therefore, shot overlap within a pass is avoided.
In another variation of the FIG. 1 method, PEC step 108 is
performed by the charged particle beam system itself, so PEC step
108 and the expose resist step 112 are combined.
FIG. 3 illustrates an example of how a dosage map 304 can be used
to show the dosage of a CP shot used to form a circular pattern 302
on a resist-coasted surface. The area in the vicinity of the circle
has been divided into a grid of squares, where each square
represents a point or sample point in the Cartesian plane where the
dosage will be calculated. The size of the grid relative to the
circle in this example is larger than would be typical, and is used
for illustration. This grid becomes a dosage map by calculating and
recording the charged particle dosage for each sample point.
Charged particle beam simulation may be used to calculate the
dosage in each grid square. The nominal dosage of the CP shot in
this example is 1.0, meaning 1.0 times a normal dosage. The blur of
the charged particle beam caused by forward scattering of the
charged particles, Coulomb effect and other physical, chemical and
electromagnetic effects causes a gradual falloff of dosage around
the edges of the circular CP shot. The resist threshold is that
dosage level above which the resist will register a pattern. If a
resist with a threshold of approximately 0.6 is used, a pattern
similar to the target pattern will be registered by the resist. In
the example of FIG. 3, the grid is too coarse to precisely
determine the shape of the pattern that will be registered by the
resist. The use of a finer grid allows a more accurate calculation
of the registered pattern, but also requires more computational
effort to calculate. Additionally, since the dosage across a single
grid square varies, any of a variety of conventions can be used in
calculating the grid dosage value. The calculated dosage for each
grid may, for example, represent the average dosage over the area
of the grid, or may represent the dosage in the lower-left corner
of the grid square, or may represent the dosage in the center of
the grid square. Some other convention may also be used. The shot
information, including shot dosage, shot shape if VSB, shot
location on the stencil if CP, partial character exposure
information if CP, and the dosage map information can be stored in
a glyph library, so that the dosage map for other shots which match
this shot's shape and dosage can be quickly accessed. Glyph
creation is, in fact, the process of calculating a dosage map for a
shot or group of shots and storing the shot information and
calculated dosage map for future use. The calculated dosage map may
be stored either as a two-dimensional matrix of dosage values as
shown in FIG. 3, or in a different format, such as a set of
instructions for creating a two-dimensional set of dosage
values.
FIG. 4 illustrates a grid map 402 showing an open arc 404. The arc
404 represents a portion of a 200 nm circular pattern, and the grid
map 402 is a portion of a grid map for the circular pattern using a
4 nm grid. The nominal shot dosage is 1.0. A resist threshold of
0.5 is used in this example. This illustrates the large number of
grid calculations that a fine grid can require even for a small
pattern. Grid sizes between 1 nm and 40 nm, in the scale of a
surface, may be useful for calculating dosages for patterns for
modern semiconductor processes. Larger grid sizes, such between 50
nm and 1 micron, may be used for calculation of longer-range
exposure effects such as backscatter and fogging, and may also be
more appropriate for manufacturing patterns for other products.
FIG. 5 and FIG. 6 illustrate how dosages for multiple shots may be
combined as an embodiment of the present disclosure. FIG. 5 shows a
two-dimensional dosage map 502 for a single rectangular VSB shot.
The calculation of the shot dosage map 502 may be accomplished
using charged particle beam simulation. The nominal shot dosage is
1.0 in this example. Use of a resist with a threshold of 0.6 will
cause a pattern similar to the rectangle to be registered on the
resist. FIG. 6 illustrates a dosage map 602 which may result from a
set of six overlapping shots onto a resist-coated target surface,
such as a reticle or a semiconductor wafer substrate. Dosage map
602 is a combination of six dosage maps of the type 502. The
nominal outlines of the six shots are shown. The combination of
dosage maps can be done by creating a dosage map 602 for the target
surface, and then combining each shot dosage map into the target
surface dosage map. The combination process involves aligning each
shot dosage map within the Cartesian coordinate space of the target
surface dosage map, then applying a mathematical operation or set
of operations, such as addition, to combine the dosage value for
each grid position of the shot dosage map into the dosage value for
the corresponding grid position in the target surface dosage map.
In this example the outlines from each of the six rectangular shots
indicate how each of the six shot dosage maps are aligned within
the Cartesian coordinate space of the target surface dosage map
602. In this example, the mathematical operation used is simple
addition. Dosage maps may also be combined using more complex sets
of mathematical operations. For example, the combination operation
could incorporate calculation of resist charging, which can cause
translation and deformation of a shot due to the negative charge
which has accumulated on the resist from temporally recent and
geometrically nearby shots. In one embodiment the created target
surface dosage map 602 may initially contain no shot information.
The target dosage map 602 may be empty, with all entries having
zero dosage, or the target dosage map 602 may be initialized with
an estimate for the long range dosage effects, such as back scatter
and fogging. In another embodiment the target dosage map 602 may be
initialized with dosages from one or more shots determined without
use of a dosage map. As can be seen from the target surface dosage
map 602, with a resist threshold of 0.6, the pattern that the
resist registers will be smoother than the union of the outline of
the individual shots. For example, the interior corners of the
unioned shot pattern will be substantially filled in, since the
dosages in these grid positions is either 0.6 or 1.0. This dosage
map 602 illustrates that the pattern registered on the resist from
this set of six shots will approximate, in the middle portion, a
constant width line angled 45 degrees with respect to the Cartesian
axes. Creation of the one-shot dosage map 502, such as by using
charged particle beam simulation, allows calculation of the dosage
map 602 by combining each of six copies or instances of the dosage
map 502 into the initial target surface dosage map 602. This may be
computationally faster than simulating the collection of six VSB
shots using charged particle beam simulation.
FIGS. 7A-D illustrate another example of combining dosage maps as
an embodiment of the present disclosure. FIG. 7A shows a desired
circular pattern 702. FIG. 7B shows a dosage map 704 of a
rectangular shot that can be used in a plurality of shots to create
the pattern 702. The nominal shot dosage for the shot represented
in the dosage map 704 is 0.7, meaning 0.7 times a normal dose. FIG.
7C shows a dosage map 706 for a square shot with a nominal shot
dosage of 0.6. FIG. 7D shows a combined dosage map 710 resulting
from the combination of three dosages maps from three overlapping
shots: a) the dosage map 704 of the rectangular shot, b) a 90
degree rotated version of dosage map 704, and c) dosage map 706 of
the square shot. If a resist with a threshold of 0.7 is used, a
pattern similar to the desired circular pattern 702 will be
registered on the resist per the combined dosage map 710. In this
example the shots represented by dosage maps 704 and 706 use a
dosage less than 1.0, so as to limit the maximum dosage to 2.0 in
the area where all three shots overlap, as shown in dosage map 710.
Some photomask production processes limit the maximum combined
dosage to values such as 2.0 times the normal dosage. FIG. 7D also
illustrates how the length of the two non-square rectangular shots
has been made larger than the diameter of the desired circular
pattern 702. The "oversizing" of these rectangles compensates for
corner rounding that may occur on these shots because of the dosage
of 0.7. As shown in dosage map 704, the dosage is less near the
edges and in the corners of the shot, due to the Gaussian dosage
fall-off near the edges of the shot. Overall, FIGS. 7A-D illustrate
how a circular pattern can be calculated using a small number of
shot dosage maps--in this case only two. Although FIGS. 7A-D
illustrate the combination of dosage maps for a circular pattern,
this method is applicable to any rectilinear or curvilinear shape
or set of shapes.
FIG. 9 illustrates in graphical form an example of a glyph. A glyph
is a dosage map calculated from one or more CP and/or VSB shots,
with each shot comprising a position and a shot dosage. The glyph
illustrated in FIG. 9 may be, for example, calculated from a shot
of a circular CP character. The glyph's two-dimensional dosage map
is displayed in FIG. 9 as a dosage graph 900. The dosage graph 900
is shown in three-dimensional isometric view, with the "Z"
dimension representing the dosage at each X, Y location. The center
of the CP shot is point 902, which is also the point of highest
calculated dosage. As can be seen, the dosage falls off in any X, Y
direction from point 902. Also shown on dosage graph 900 is a
resist threshold 904, which is the dosage above which resist
coating a surface would register a pattern if the resist were to be
exposed with only this shot. The portion of the dosage graph which
is above the resist threshold 904 is marked as graph portion 906.
The portion of glyph 900 which will result in a registered pattern
area on a resist-coated surface is thus the projection of graph
portion 906 onto the X-Y plane. As can be seen from FIG. 9, the
registered pattern area created by flattening graph portion 906 is
circular or nearly circular. The glyph calculated from the circular
CP character and represented by the dosage graph 900 is therefore
circularly symmetric or nearly circularly symmetric, and will
produce a circular or nearly circular registered pattern area on
the resist-coated surface.
FIG. 2 illustrates an exemplary conceptual flow diagram 200 of a
method for manufacturing a photomask according to the current
disclosure. There are three types of input data to the process:
stencil information 218, which is information about the CP
characters on the stencil of the charged particle beam system;
process information 236, which includes information such as the
resist dosage threshold above which the resist will register a
pattern; and a computer representation of the desired pattern 216
to be formed on the reticle. In addition, initial optional steps
shown by steps 202-212 involve the creation of a library of glyphs.
The first step in the optional creation of a library of glyphs is
VSB/CP shot selection 202, in which one or more VSB or CP shots,
each shot with a specific dosage, are combined to create a set of
shots 204. The set of shots 204 may include overlapping VSB shots
and/or overlapping CP shots. The VSB/CP shot selection step uses
the stencil information 218, which includes information about the
CP characters that are available on the stencil. The set of shots
204 is simulated in step 206 using charged particle beam simulation
to create a dosage map 208 of the set of shots. Step 206 may
include simulation of various physical phenomena including forward
scattering, resist diffusion, Coulomb effect and etching. The
result of step 206 is a two-dimensional dosage map 208 which
represents the combined dosage from the sets of shots 204 at each
of the grid positions in the map. The dosage map 208 is called a
glyph. In step 210 the information about each of the shots in the
set of shots, and the dosage map 208 of this additional glyph is
stored a library of glyphs 212. In one embodiment, a set of glyphs
may be combined into a type of glyph called a parameterized
glyph.
The required portion of the flow 200 involves creation of a
photomask. In step 220 a combined dosage map for the reticle or
reticle portion is calculated. Step 220 uses as input the desired
pattern 216 to be formed on the reticle, the process information
236, the stencil information 218, and the glyph library 212 if a
glyph library has been created. In step 220 a reticle dosage map
may be created, into which shot dosage information, for example a
shot dosage map, will be combined. In one embodiment the reticle
dosage map may be initialized to zeros. In another embodiment, the
grid squares of the reticle dosage map may be initialized with an
estimated correction for long-range effects such as backscattering,
fogging, or loading, a term which refers to the effects of
localized resist developer depletion. In another embodiment, the
reticle dosage map may be initialized with dosage information from
one or more glyphs, or from one or more shots which have been
determined without use of a dosage map. Step 220 may involve VSB/CP
shot selection 222 or glyph selection 234, or both of these. If a
VSB or CP shot is selected, the shot is simulated using charged
particle beam simulation in step 224 and a dosage map 226 of the
shot may be created. The charged particle beam simulation may
comprise convolving a shape with a Gaussian. The convolution may be
with a binary function of the shape, where the binary function
determines whether a point is inside or outside the shape. The
shape may be an aperture shape or multiple aperture shapes, or a
slight modification thereof. In one embodiment, this simulation may
include looking up the results of a previous simulation of the same
shot, such as when using a temporary shot dosage map cache. In
another embodiment, the shot dosage information may be represented
in some way other than a dosage map, where this other
representation allows the shot dosage information to be combined
into the reticle dosage map. Both VSB and CP shots may be allowed
to overlap, and may have varying dosages with respect to each
other. If a glyph is selected, the dosage map of the glyph is input
from the glyph library. In step 220, the various glyph dosage maps
and the shot information such as shot dosage maps are combined into
the reticle dosage map. In one embodiment, the combination is done
by adding the dosages. Using the resulting combined dosage map and
the resist information 236, a reticle pattern may be calculated. If
the reticle image matches the desired pattern 216 within a
pre-determined tolerance, then a combined shot list 238 is output,
containing the determined VSB/CP shots and the shots constituting
the selected glyphs. If the calculated reticle image does not match
the target image 216 within a predetermined tolerance as calculated
in step 220, the set of selected CP shots, VSB shots and/or glyphs
is revised, the dosage maps are recalculated, and the reticle
pattern is recalculated. In one embodiment, the initial set of
shots and/or glyphs may be determined in a correct-by-construction
method, so that no shot or glyph modifications are required. In
another embodiment, step 220 includes an optimization technique so
as to minimize either the total number of shots represented by the
selected VSB/CP shots and glyphs, or the total charged particle
beam writing time, or some other parameter. In yet another
embodiment, VSB/CP shot selection 222 and glyph selection 234 are
performed so as to generate multiple sets of shots, each of which
can form a reticle image that matches the desired pattern 216, but
at a lower-than-normal dosage, to support multi-pass writing.
The combined shot list 238 comprises the determined list of
selected VSB shots, selected CP shots and shots constituting the
selected glyphs. All the shots in the final shot list 238 include
dosage information. In step 240, proximity effect correction (PEC)
and/or other corrections may be performed or corrections may be
refined from earlier estimates. Step 240 uses the combined shot
list 238 as input and produces a final shot list 242 in which the
shot dosages have been adjusted. The final shot list 242 is used by
the charged particle beam system in step 244 to expose resist with
which the reticle has been coated, thereby forming a pattern 246 on
the resist. In step 248 the resist is developed. Through further
processing steps 250 the reticle is transformed into a photomask
252.
FIGS. 8A & B illustrate an example of a parameterized glyph.
The dosage map 802 illustrated in FIG. 8A is for a rectangular shot
804 of width 812, or eight grid units in this example. The two
vertical lines 806 and 808 define a region of the dosage map which
is of width 810, or four grid units in this example. Within this
region 810 of the dosage map 802, all grid squares in each row have
identical dosage values. FIG. 8B illustrates a dosage map 820 for a
rectangular shot 824 of width 832, or twelve grid units in this
example. The dosage map 820 is similar to the dosage map 802,
including the dosage values of the grid squares, except that
between vertical lines 826 and 828, dosage map 820 contains four
more grid columns than the dosage map 802 contains between lines
806 and 808. This "stretchable" portion of the dosage map 820 is of
width 830, or eight grid units in this example. By identifying a
stretchable or parameterizable region where the dosages are
identical along the stretchable dimension, such as the region
between lines 806 and 808 of FIG. 8A or between lines 826 and 828
of FIG. 8B, a dosage map for a rectangular shot of the same height
as shots 804 and 824 can be generated for shots with any width
greater than 812. Limitations of the charged particle beam system
may further restrict the size of the rectangular shots for which
this method can be used to generate a dosage map. In other
embodiments, a repeated dosage pattern in the dosage map may allow
dosage maps to be generated for single shots or groups of shots of
only discrete lengths, rather than of a continuous length such as
the example of FIGS. 8A & B. This example shows how a dosage
map for a parameterized glyph may be generated. In other
embodiments, other dimensions may be parameterized, such as height
or diameter.
While the specification has been described in detail with respect
to specific embodiments, it will be appreciated that those skilled
in the art, upon attaining an understanding of the foregoing, may
readily conceive of alterations to, variations of, and equivalents
to these embodiments. These and other modifications and variations
to the present methods for fracturing, creating glyphs and
manufacturing a surface may be practiced by those of ordinary skill
in the art, without departing from the spirit and scope of the
present subject matter, which is more particularly set forth in the
appended claims. Furthermore, those of ordinary skill in the art
will appreciate that the foregoing description is by way of example
only, and is not intended to be limiting. Steps can be added to,
taken from or modified from the steps in this specification without
deviating from the scope of the invention. In general, any
flowcharts presented are only intended to indicate one possible
sequence of basic operations to achieve a function, and many
variations are possible. Thus, it is intended that the present
subject matter covers such modifications and variations as come
within the scope of the appended claims and their equivalents.
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